1. Technical Field
The present invention relates to a lighting device for lighting a discharge lamp, a light source device including the lighting device and a discharge lamp, a projector including the light source device, and a discharge lamp lighting method.
2. Related Art
A discharge lamp which includes an arc tube having a light emission portion and a pair of sealing portions such as extra-high pressure mercury lamp is known. The light emission portion has a discharge space accommodating a pair of electrodes. The sealing portions extend in directions away from each other with the light emission portion located between the sealing portions, and each of the sealing portions contains an electrode extension line connected with the corresponding electrode. Light emitting substances containing mercury, rare gas and the like are sealed into the discharge space of the discharge lamp. The mercury adheres to one of the pair of the electrodes disposed within the discharge space, which electrode is more easily cooled when the discharge lamp is turned off. Discharge phenomenon until stabilization of arc discharge is affected by this adhesion of mercury.
More specifically, in the initial stage of arc discharge, first, arc discharge on mercury having low work function starts, and then arc discharge on the electrodes made of tungsten starts after the temperature of tungsten reaches a temperature sufficient for thermionic emission (thermionic emission temperature). The actions during this period are affected by the condition of mercury adhesion and the corresponding electron emission. In case of electron emission which does not agree with mercury adhesion under the condition that the temperature of the tungsten electrode does not reach the thermionic emission temperature, mixture of glow discharge and arc discharge is generated. In this case, the tungsten electrodes are subject to cathode sputtering, thereby producing excessive tungsten condition inside the container of the light emission portion. This condition leads to blacking phenomenon which causes adhesion of tungsten to the inner wall of the container of the light emission portion, and thus shortens the life of the discharge lamp.
A high-pressure discharge lamp lighting device is known as a lighting control device for controlling lighting of the discharge lamp. This high-pressure discharge lamp lighting device applies AC rectangular wave voltage at the start of light emission of the discharge lamp, applies DC voltage at the time of transition from glow discharge to arc discharge, and applies AC rectangular wave voltage at the time of steady lighting (For example, see JP-A-2004-39391).
The high-pressure discharge lamp lighting device disclosed in JP-A-2004-39391 applies high-frequency AC voltage at the start of light emission of the discharge lamp to supply current to the respective electrodes according to the polarity inversion. Then, the high-pressure discharge lamp lighting device applies DC voltage to increase the temperatures of the respective electrodes, thereafter applies AC voltage to achieve steady lighting in a stable manner.
When AC current is supplied to the discharge lamp in the starting period of the discharge lamp in the manner shown in JP-A-2004-39391 under the condition in which mercury is only adhering to the electrode easily cooled during turned-off time of the discharge lamp, asymmetric current (half-wave current) drive is easily produced in which continuity of only one-directional current is recognized between the electrode having mercury adhesion and the electrode having no mercury adhesion. In this case, lighting startability of the discharge lamp lowers. When the period of the asymmetric current drive is long, electrons emitted from one electrode easily collide with the tube wall of the light emission portion. As a result, the constituent material of the tube wall (such as glass) evaporates. The constituent material evaporated in this manner changes the interior environment of the light emission portion, and causes considerable deterioration of the discharge lamp such as exhaustion of the electrodes. Therefore, a lighting device capable of improving lighting startability of a discharge lamp has been demanded.